Actress Lucy Liu has announced the arrival of her baby boy, which she had through the help of a surrogate. Although surrogacy is not very common, there are many reasons why women and couples may chose surrogates to be part of their fertility treatment.

The 46-year old Liu made the announcement through Instagram, where she posted a photo of herself holding her son. A representative for Liu said the actress had a gestational carrier for her son, according to the Los Angeles Times. Liu joins others celebrities, including Jimmy Fallon and his wife Nancy, Sarah Jessica Parker and husband Matthew Broderick, and actress Nicole Kidman and husband Keith Urban, who have all used surrogates to carry their children.

A gestational carrier (or gestational surrogate) is a woman who carries a child for another couple, but the carrier not the biological mother of the child, according to the American Society for Reproductive Medicine (ASRM). Instead, the egg from the biological mother and sperm from the father are combined in a lab dish (through a process called in vitro fertilization), before the fertilized embryo is implanted in the gestational carrier. (Couples who turn to a surrogate for carrying a pregnancy may use their own eggs and sperm, or donor cells.)

The use of a gestational carrier in fertility treatment is still not very common. Dr. Tomer Singer, a reproductive endocrinologist at Lenox Hill Hospital in New York City, estimates that about 5 percent of the couples he sees with fertility problems use a gestational carrier. [Future of Fertility Treatment: 7 Ways Baby-Making Could Change]

Some of the reasons why a couple may need to use a gestational carrier include:

Older women may be more likely than younger women to need a gestational carrier, because some the risk of some conditions that affect the ability to carry a pregnancy such as uterine fibroids, and recurrent miscarriages increase with age, Singer said. Older women may also be more likely to have chronic conditions that would make pregnancy risky, such as heart, lung or thyroid conditions.

Singer noted that laws regarding gestational carriers differ by state. Some states, such as California, allow gestational carriers to be paid, but other states do not allow compensation.